Mach-Zehnder modulators achieve amplitude modulation based upon the phase difference between two arms of Mach-Zehnder modulator being proportional to a difference in voltage signals applied to the two arms. The Mach-Zebnder structure allows for this phase modulation to be convened into amplitude modulation. When the input voltage signals are AC-coupled, a DC bias is required in order to set the operating point of the modulator. Due to physical processes in the modulator, the DC bias voltage required for proper operation is time-varying. In order to maintain the proper bias condition, a control circuit is needed that monitors the output of the modulator, and corrects for the time varying bias requirement. For On-Off Keying, such an Automatic Bias Control (ABC) circuit is well-known, and in use in many implementations (see, for example, “Using the Lithium Niobate Modulator: Electro-Optical and Mechanical Connections” Lucent Technologies Application Note TN98-004LWP, April 1998.) When advanced line codes are used (see for example commonly owned U.S. Patent Publication No. US-2004-0247324-A1, filed on even date herewith, which is incorporated herein in its entirety), however, the conditions that permit the use of the conventional ABC circuit are not met, and a new, more sophisticated ABC circuit is needed.
Accordingly, it would be desirable to provide a method and apparatus for automatically biasing a class of advanced line codes.